Cells from xeroderma pigmentosum complementation group C (XP-C) patients exposed to ultraviolet light (UV, 254 nm) repair DNA in large unique chromatin regions (at least 30 to 80 kb) that represent about 10% of the genome. The remainder of the genome is not repaired. The proposed research is aimed at understanding this domain-oriented DNA excision repair process and its significance, and its relation to excision repair in normal human cells. These are steps to defining the repair defect in XP-C cells and its relation to the high incidence of cancer in XP patients. They are also steps to understanding the significance of this residual repair activity to the diminished XP phenotype in XP-C patients and the relative resistance of nondividing XP-C cells to the lethal effects of UV. To date, six repair domains have been identified and partially characterized by screening a DNA fraction enriched in the preferentially repaired DNA with gene-specific probes. This DNA fraction also has more copies of a mildly repetitive nuclear matrix associated DNA sequence (MAR) found in transcription domains than does an unrepaired DNA fraction. The selective repair process will be characterized with respect to the genetic content and physical structure of the unique domains and the relation to normal excision repair. These goals will be accomplished by identifying other repair domains, by studying some repair domains in detail and by determining the repair rate of specific DNA fragments contained within domains in normal and other repair defective cells. The proto-oncogenes are targeted in the screening procedure to identify other repair domains because their chromatin location may play a fundamental role in their activation to an oncogenic form. Details of domain structure will be studied in the large domain (at least 50 to 80 kb) containing the relatively small beta-actin gene (3.5 kb). Domain termini and size will be determined by mapping the repair activity in neighboring genomic regions, copies of which are obtained from a cosmid library. The component parts of the domain will examined for transcriptional activity and putative MAR sequences to test the hypothesis that repaired domains are associated with transcription. The repair rates of specific DNA fragments in normal and some repair defective cells will be studied to determine the relation to normal repair mechanisms and the generality of "repair domains" throughout the human population. The significance of domain-oriented repair will be assessed by studying the effect of UV and subsequent excision repair on transcription of domain-associated genes in normal and XP-C strains and in UV-sensitive XP-A strains that have limited repair equivalent to that in XP-C cells but at random rather then specific genomic locations.